Fix incorrect date offset calculation in rust

[AetherUnbound]

Description

Fixes #916

I dug into the rust parsing code a bit and saw that the ordinal calculation might be why 2026W36 was getting the error ValueError: day is out of range for month. 2026W36 should be August 31, 2026, which becomes ordinal day 243.

Looking at the MONTHS_OFFSETS for non-leap year (2026):
[0]==-1, [1]==0, [2]==31, [3]==59, [4]==90, [5]==120, [6]==151, [7]==181, [8]==212, [9]==243, [10]==273, [11]==304, [12]==334, [13]==365

The original loop: for i in 1..14:

• i=1: ord < MONTHS_OFFSETS[0][1] → 243 < 0? No
• i=2: ord < MONTHS_OFFSETS[0][2] → 243 < 31? No
• ...
• i=8: ord < MONTHS_OFFSETS[0][8] → 243 < 212? No
• i=9: ord < MONTHS_OFFSETS[0][9] → 243 < 243? No ← This is the problem!

Since 243 < 243 is false, it continues to:

• i=10: ord < MONTHS_OFFSETS[0][10] → 243 < 273? Yes

At this point, the original code would execute:

let day = ord as u32 - MONTHS_OFFSETS[leap][i - 1] as u32;  // i=10, so i-1=9
let month = (i - 1) as u32;  // month = 9 (September, 1-based)

So: day = 243 - MONTHS_OFFSETS[0][9] = 243 - 243 = 0

This gives us month=9, day=0, which is September 0th - an invalid date, hence the "day is out of range for month" error.

Changing the comparison to be <= means that September (9) is matched appropriately as the month offset, so when it becomes 1-based with let month = (i - 1) as u32;, it becomes August once more.

I added a new test case for this as well in order to verify the functionality! I'm no rust expert though so this may be completely off base 😅

Shoutout to @d3jawu for helping walk me through some of the rust pieces 😊

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

[arthurnw]

Great test case!

[codspeed-hq]

CodSpeed Performance Report

Merging #918 will not alter performance

_{Comparing AetherUnbound:fix/week-formatting-bug (c0901d6) with master (0e92991)}

Summary

✅ 1 untouched